An art is known for adjusting a gas pressure inside a reservoir reserving oil by forming a communicating path in a cap of the reservoir and arranging a valve component. For example, Japanese Patent Application Laid-open No. H9-14475 discloses a cap for a fuel tank. As the valve component arranged on the cap, an air intake valve that introduces air into the fuel tank from an outside and an exhaust valve that discharges a gas inside the fuel tank to the outside are formed. Consequently, the gas pressure inside the fuel tank is maintained within a predetermined range.